1. Technical Field
The present invention relates to fuel metering valves, and in particular, to highly responsive valves suitable for use in high speed active combustion control systems.
2. Description of the Related Art
Many modern gas turbines have combustion cans with low “NOX” burners, that is burners designed to produce low emissions of oxides of nitrogen known to be harmful to the environment. These burners use various lean fuel/air mixing techniques to reduce the levels of nitrogen oxides exhausted. Altering the fuel to air ratio to reduce NOX emissions can cause “screech” or instability in the combustion such that the burner flame is inconsistent or unsustainable. The unstable burning generates pressure fluctuations in the combustion can. These pressure fluctuations cause oscillating waves in the combustion can on the order of several hundred cycles per second. This high frequency vibration can cause rapid breakdown of the combustion components which can send particles or other debris to the turbine blades and thereby cause severe structural damage to the turbine.
Sophisticated combustion control systems have been developed to continuously monitor and actively stabilize the combustion of modern gas turbines to avoid or minimize these adverse affects on the turbine. These systems usually include high speed pressure transducers located, for example, in the combustion can to sense the pressure oscillations arising from the unstable burning. The transducers provide pressure signals to a control computer which processes the signals according to various algorithms to control various combustion components or parameters to counteract the pressure oscillations. Typically, this involves pulsing the fuel sent to the combustion can at very high rates commensurate with the frequency of the pressure oscillations. This is ordinarily accomplished by rapidly operating the fuel metering valve.
Modern gas turbine engines used for utility power generation are very large and capable of a continuous power output between 200-500 megawatts. Such high output requires significant fuel consumption on the order of 200-400 gallons per minute, which must be mixed with air and delivered to the combustion can at a somewhat astonishing rate of roughly 1.25 million cubic feet per minute. The metering valve used in such a system must thus be able to meter very high flow volumes and by operable at very high rates to counter the high frequency pressure oscillations.
Typical metering valves do not meet the extreme performance requirements in gas turbine active combustion control systems. Even the fuel metering valves used in existing active combustion control systems lack the desired responsiveness and efficiency.
Accordingly, an improved fuel metering valve capable of better sustained operation in an active combustion control system of a gas turbine engine is needed.